Unless otherwise indicated herein, the description provided in this section is not itself prior art to the claims and is not admitted to be prior art by inclusion in this section.
A cellular wireless network may include a number of base stations that radiate to define wireless coverage areas, such as cells and cell sectors, in which user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices (whether or not technically operated by a human user), can operate. In turn, each base station may be coupled with network infrastructure, including one or more gateways and switches, that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or a packet-switched network such as the Internet for instance. With this arrangement, a UE within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs.
In general, a cellular wireless network may operate in accordance with a particular radio access technology or “air interface protocol,” with communications from the base stations to UEs defining a downlink or forward link and communications from the UEs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA (e.g., Long Term Evolution (LTE) or Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), and Global System for Mobile Communications (GSM), among others. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover of UEs between coverage areas, and functions related to air interface communication.
In accordance with the air interface protocol, each coverage area may operate on one or more carrier frequencies for carrying communications between the base station and UEs. Each carrier frequency could be frequency division duplex (FDD), in which the downlink and uplink operate on separate frequency channels, or time division duplex (TDD), in which the downlink and uplink operate on a shared frequency channel and are distinguished from each other over time. Further, each carrier may be structured to define certain air interface resources for carrying communications.
In a representative OFDMA network, for instance, the downlink in each coverage area is mapped over frequency and time into an array of resource elements in which the base station can transmit data to UEs. In particular, the downlink is divided over frequency into a range of closely-spaced orthogonal subcarriers and is divided over time into a continuum of symbol time segments, thereby defining an array of resource elements each centered on a respective subcarrier and spanning a respective symbol time segment. With this arrangement, as the base station has data to transmit to UEs, the base station may transmit the data in particular resource elements to the UE.
By way of example, in accordance with the LTE protocol, the downlink of each carrier could span a frequency bandwidth such as 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz, and the frequency bandwidth is divided into 15 kHz subcarriers (subcarriers spaced apart from each other by 15 kHz). Further, the air interface is divided into a continuum of 10-millisecond (ms) frames, and each frame is divided into ten 1 ms sub-frames or transmission time intervals (TTIs), some or all of which may be used for the downlink. Each 1 ms downlink TTI is then further divided into 14 symbol time segments, each spanning 66.7 microseconds plus an added 4.69 microsecond guard band (cyclic prefix). With this arrangement, each TTI thus defines an array of resource elements, each centered on a 15 kHz subcarrier and spanning a symbol time segment, and each such resource element may effectively carry a single orthogonal frequency division multiplexing (OFDM) symbol representing communication data.
In each TTI, certain ones of these resource elements on the downlink are reserved for carrying particular types of communications. For instance, particular resource elements distributed throughout the downlink bandwidth are reserved for carrying a reference signal that UEs can detect and measure as a basis to determine the quality (e.g., strength) of coverage. Further, across the bandwidth, resource elements in the first one, two, or three symbol segments are reserved to define various control channels, such as a physical downlink control channel (PDCCH) in which the base station provides control signals such as resource allocation directives and the like. And the remaining symbol segments in each TTI are then generally reserved to define a physical downlink shared channel (PDSCH) for carrying data to UEs in accordance with the base station's resource allocation directives.
In addition, the resource elements in each TTI are grouped into physical resource blocks (PRBs), each spanning 12 resource elements (180 kHz) in the frequency domain and 7 resource elements in the time domain, thus defining an array of 84 resource elements—although some may be reserved for special use as noted above. Thus, depending on the downlink bandwidth, the air interface may support transmission on a number of such downlink resource blocks in each TTI. For instance, a 5 MHz carrier may support 25 resource blocks in each TTI, whereas a 15 MHz carrier may support 75 resource blocks in each TTI.
When a UE powers on or otherwise enters into coverage of a base station, the UE and base station may engage in signaling with each other to establish an agreed air interface connection, such as a Radio Resource Control (RRC) connection, through which the base station will then serve the UE. Further, the UE may engage in an attachment or registration process via the base station with the network, which may involve the network authenticating and authorizing the UE and establishing one or more network connections for carrying communications between the UE and one or more transport networks. Having an established air interface connection with the base station, the UE may then be considered to be operating in a connected mode.
With the UE operating in the connected mode, as the base station receives data for transmission to the UE, the base station may select particular air interface resources to carry the data, and the base station may transmit to the UE a resource allocation directive specifying the selected resources and transmit the data to the UE in the selected/specified resources. In LTE, for instance, the base station may select particular PRBs to carry the data in a given TTI, the base station may transmit to the UE in the PDCCH of that TTI a downlink control information (DCI) message that specifies the PRBs, and the base station may transmit the data to the UE in the specified PRBs. The UE may thus read the DCI to determine the PRBs carrying data to the UE, and the UE may then read the data from the specified PRBs.
In addition, in the connected mode, the UE may regularly monitor the quality of the base station's coverage and provide the base station with channel-quality reports, to enable the base station to adapt its air interface transmission with the UE. In particular, the UE may regularly establish and report to the base station a channel quality indication (CQI) based on coverage quality (e.g., signal-to-noise ratio) and other channel attributes, and the base station may map the UE's reported CQI to an appropriate modulation and coding scheme (MCS) to be used for air interface communication with the UE, using a predefined CQI-to-MSC mapping table. When the base station has data to communicate to the UE, the base station may then specify the determined MCS in its DCI to the UE and may engage in the transmission accordingly.
The MCS selected by the base station based on the UE's CQI defines a coding rate and a modulation scheme for communicating data from the base station to the UE. In particular, the coding rate defines a rate of usable data communication, taking into account error-correction coding added to help overcome errors in transmission. And the modulation scheme then defines how data will be modulated for transmission on air interface resources to the UE, including how many bits of a coded data stream (sequence of bits) the base station can transmit in each air interface resource, such as with each OFDM symbol. Examples of modulation schemes include (i) Quadrature Phase Shift Keying (QPSK), which represents 2 bits per symbol, (ii) 8PSK, which represents 3 bits per symbol, (iii) 16 Quadrature Amplitude Modulation (16QAM), which represents 4 bits per symbol, and (iv) 64QAM, which represents 6 bits per symbol.
In general, a lower-order MCS, using a lower coding rate (more error-correction data) and/or a modulation scheme in which each air interface resource represents fewer bits, may be more robust and error-tolerant and thus more suitable when the UE's channel conditions are poor. Whereas, a higher-order MCS, using a higher coding rate (less error-correction data) and/or a modulation scheme in which each air interface resource represent a greater number of bits, may be less robust but may provide higher throughput and may thus be more suitable when the UE's channel conditions are good.